New data from Netflix shows a noticeable drop in speed for customers who stream movies through Verizon FiOS and Comcast, at the same time the speed from other Internet service providers has improved or held steady.

Comcast’s average speeds for Netflix users have dropped dramatically in just a handful of months. From January through September of 2013, Comcast bounced around between 2 Mbps and about 2.13 Mbps. But starting in October, their performance fell and by January of this year, their average was closer to 1.5 Mbps. FiOS saw a similar, though not quite as precipitous, drop, from a high around 2.2 Mbps to their current low of about 1.8.

Consumerist says part of the reason may be that Netflix has changed the way it measures these things. But that can’t be the complete explanation because the streaming speed from other Internet service providers has improved or held steady.

Consumerist says there’s no real way to figure out what’s going on. Peter Bright wrote on Ars Technica that it’s not possible for consumers to tell the difference “between poor performance that’s a ‘natural’ result of congestion and poor performance that’s an ‘artificial’ result of traffic shaping and prioritizing.”

Could an appeals court’s decision to strike down the net neutrality rule at Verizon’s request have something to do with this? (Money Talks News founder Stacy Johnson wrote a post last month explaining just how important that decision is — for instance, it could mean higher costs for Netflix, which could mean higher costs for you.)

There’s been speculation about that. Consumerist says:

There is now no active rule requiring ISPs to treat different internet traffic the same way. So major networks might actually be getting slower … or they might be throttling some of the traffic moving through them.

Big providers have been accused of doing such things in the past.

But Jon Brodkin, in another Ars post, said the slowing of Verizon’s Netflix service began before the court’s ruling last month. He also notes that Comcast is still bound by net neutrality rules because of conditions included in its purchase of NBCUniversal. (You can read Verizon’s response in Brodkin’s post. Comcast hadn’t issued a comment.)

We do know that in the past, Verizon and Comcast have each been involved in disputes with Internet bandwidth providers, and that these disputes can prevent peering infrastructure from being upgraded. This affects all traffic, but it places a more noticeable toll on streaming video because of how much bandwidth it requires. …

After this story published, one commenter pointed out that the declines in performance came after Netflix started delivering its so-called “Super HD” and 3-D video to all customers, even those whose ISPs are not members of Open Connect. [Comcast and Verizon are not.] This may have increased the traffic load.

In fact, Netflix has said that failure to sign up for Open Connect could result in lower speeds, writes Washington Post tech writer Brian Fung. “Some have argued that the [ISP speed] ranking serves two purposes: to give an account of streaming speeds, yes, but also to shame providers that haven’t signed on with Open Connect.” (This TechCrunch post explains what Open Connect does.)

He also says Netflix has found no indication that Verizon is throttling bandwidth.

Meanwhile, let’s not forget that Comcast has offered to buy Time Warner Cable for $45 billion.

Update: As for Netflix’s DVD-by-mail service, I told you about how I was no longer getting movies a day after they were placed in the mail. It got so bad, movies were arriving five to seven days later, making the service essentially worthless. I called customer service and was told the post office was to blame. After my second complaint, the service has improved to three or fewer days. Let’s see how long that keeps up.

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